LT3477 datasheet(8/26 Pages) AD

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LT3950

Rev. 0

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PIN FUNCTIONS

ISP: Kelvin connect this pin to the high side of the LED

current feedback sense resistor. Current in the sense

resistor is 250mV/ RSENSE while the CTRL pin voltage

exceeds 1.2V. It varies as (CTRL – 200mV)/(4 • RSENSE)

if the voltage at the CTRL pin is between 200mV and 1.2V.

ISN: Kelvin connect this pin to the low side of the LED

current feedback sense resistor, see ISP for more details.

If the voltage difference VISP – VISN ever exceeds 700mV

(typ.), switching stops and the part re-enters soft-start.

The PWMTG pin goes high to disconnect the load, and

the part signals an overcurrent event.

FB: Output Voltage Feedback Pin. This pin is used for

output voltage regulation and limiting. Tie this pin to a

resistive voltage divider from the output voltage. When

the voltage at FB approaches 1.2V, the control loop will

reduce switch current to regulate output voltage such that

FB remains around 1.2V. If LED current falls below 10%

(typ.) of full scale while the output voltage is in regulation,

an LED Open event is signaled. If the voltage at FB exceeds

1.3V (typ.), PWMTG is driven high, switching stops, and

the device signals an overvoltage event. If the voltage at

FB falls below 300mV (typ.) after soft-start has finished,

an LED Short event is signaled at FAULT, PWMTG is driven

high, switching stops and the part re-enters soft-start. See

the Applications section for more info on the use of the

FB pin for general applications.

CTRL: Analog Alternative to PWM Dimming. Tie to INTVCC

to set RSENSE current to full scale. Current in RSENSE varies

as (CTRL – 200mV)/(4 • RSENSE) when the voltage at the

CTRL pin varies from 200mV to 1.2V.

VC: An internal error amplifier node used for compensa-

tion. Stabilize the loop by connecting a capacitor or RC

network between this pin and ground.

PWM: Pulse Width Modulation (PWM) Dimming Generator

Control Pin. Connect an analog signal to this pin to use

the internal exponential scale dimming PWM generator.

When the voltage at this pin remains between 0.2V and

1.2V, the duty ratio of the internal PWM generator will

vary with the pin voltage. A linear ramp of voltage on the

PWM pin between 0.2V and 1.2V and lasting many PWM

dimming cycles (set by RP) will result in an exponentially

increasing PWM duty ratio. An external PWM signal can

also drive this pin directly if the ON and OFF voltages are

above 1.3V and below 100mV, respectively.

SYNC/SPRD: Switch Clock Synchronize / Spread

Spectrum. Tie this pin to INTVCC to enable internal spread

spectrum frequency modulation. An external clock can

also drive this pin to synchronize switching. Choose RT

such that the LT3950 clock would be close to the external

clock, then drive this pin with that clock.

INTVCC: Voltage Supply Used by Internal Circuitry. Tie a

1µF capacitor between this pin and ground. This pin is

sometimes used as a reference voltage for other pins,

however this pin is not intended for use as a power source

for external loads, and connecting it to any external load

may interfere with operation of the system. It is not

recommended to connect INTVCC except as directed to

LT3950.

RT: Connect a resistor between this pin and ground to set

the switching frequency. Do not connect anything other

than a resistor to this pin or the system may not function

correctly.

RP: Connect a resistor between this pin and ground to set

the PWM dimming generator frequency. Connect this pin

to INTVCC to synchronize the PWM dimming clock to the

switching clock (fPWM = fSW/4096).

FAULT: Open-Drain Fault Indication Pin Indicating Short

LED, Open LED, Overvoltage and Overcurrent Faults. Tie

this pin through a 100k resistor to VIN or INTVCC, or use

it as an open drain signal. LT3950 pulls this pin low to

signal all reported fault events.

EN/UVLO: Enable/Undervoltage Lockout . When the volt-

age at this pin falls below 1.25V (typ.), with approximately

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